Acid labile H.sup.+ K.sup.+ ATPase inhibitors also named as proton pump inhibitors are for instance compounds known under the generic names omeprazole, lansoprazole, pantoprazole, pariprazole, leminoprazole and others.
These active substances are useful for inhibiting gastric acid secretion in mammals and especially in man. In a more general sense, they may be used for prevention and treatment of gastric-acid related diseases in mammals and especially in man, including e.g. reflux oesophagitis, gastritis, duodenitis, gastric ulcer and duodenal ulcer. Furthermore, they may be used for treatment of other gastrointestinal disorders where gastric acid inhibitory effect is desirable e.g. in patients on NSAID therapy, in patients with Non Ulcer Dyspepsia, in patients with symptomatic gastro oesophageal reflux disease, and in patients with gastrinomas. They may also be used in patients in intensive care situations, in patients with acute upper gastrointestinal bleeding, pre- and postoperatively to prevent aspiration of gastric acid and to prevent and treat stress ulceration. Further, they may be useful in the treatment of psoriasis as well as in the treatment of Helicobacter infections and diseases related to these.
The active compounds are, however, susceptible to degradation/transformation in acidic reacting and neutral media. The degradation is catalyzed by acidic reacting compounds. The active compounds are stabilized with alkaline reacting compounds. Thus, the active substance being a proton pump inhibitor is best protected by an enteric coating layer. There are different enteric coating layered preparations of omeprazole as well as other proton pump inhibitors described in the prior art, see for example U.S. Pat. No. 4,786,505 (AB Hassle).
There has been a demand for a formulation with a rapid dissolution and a quick onset of action, furthermore a formulation which is pleasant to take for the patient and also which is suitable for patients with swallowing difficulties (dysphagia). There are a number of dosage forms that hold a good deal of promise in administering proton pump inhibitors. However, it has been difficult to find a vehicle which can satisfy all of many and some times conflicting needs and desires for such a dosage form.
One possible vehicle for administration of these active agents is effervescent tablets. Effervescence provides generally some measure of taste-masking. Prior to being taken by the patient, an effervescent composition is dissolved and/or dispersed in for example an aqueous medium, such as drinking water. Dissolution and/or dispersion takes place rapidly, with effervescence to give an agreeable presentation of the drug, particularly for patients who do not like tablets or find difficulty in swallowing tablets.
Effervescent compositions usually contain, in addition to the active ingredient, a source of carbon dioxide (such as an alkaline carbonate or bicarbonate) and an acid (such as for instance citric acid). The use of an acid in effervescent compositions in which the active ingredient is an acid labile substance such as an acid susceptible proton pump inhibitor presents a problem due to the instability of the proton pump inhibitor in the presence of acid.
Replacement of citric acid by monosodium citrate still fails to give a satisfactory level of stability of an acid labile histamine H.sub.2 -antagonist, whilst replacement of citric acid by disodium citrate results in insufficient effervescence and a prolonged dissolution time. EP 233853 proposes a mixture of monosodium citrate and disodium citrate as a solution to the problem. GB 2 219 940 A, proposes replacement of citric acid or the mixture of citrates proposed in EP 233853 by a monoalkalimetal citrate (monosodium citrate).
Effervescent tablets containing acid-sensitive agents have been manufactured by coating the acidic particles in the acid-base couple with a coating of a base to separate the pharmaceutically active substance, i.e. the acid-sensitive agent, from the acid of the effervescence, see for instance WO 94 21,239. The proposed solution results in that the active drug comes into contact with the resulting buffer when dissolving the tablet. Thus, the active drug must be stable in that buffer at the given pH. Furthermore, if the active drug has a bad taste, there will be problems to mask it. (For instance, omeprazole is such a compound that has a strongly bitter taste).
Another way to make effervescent tablets containing acid-labile drugs, such as erythromycine, has been proposed as described in U.S. Pat. No. 4,289,751. The active substance is incorporated in the effervescent tablet, in intimate contact with the effervescing acid-base couple. The effervescent tablet is then coated with an enteric coating polyrner. The aim of the preparation is that the tablet will be protected from the strongly acidic environment in the stomach by the enteric coating layer during the passage thereof. In the small intestines, the enteric coating layer is dissolved and the effervescent effect takes place in the intestines. One drawback with such a dosage form is that patients can experience problems due to the carbon dioxide liberated inside the gastrointestinal channel. Another drawback is varying residence time in the stomach before the tablet can arrive to an environment where the active substance can be dissolved, absorbed and can exert its effect
Korean pat. appl. No. 93-17902 proposes another composition comprising an enteric coated tablet with an effervescent mixture layer inside the enteric coating. Also Korean pat. appl. No. 94-3190 describes a formulation of omeprazole with an effervescent mixture inside the enteric coating.
A way to circumvent the problems associated with the composition proposed in U.S. Pat. No. 4,289,751, i.e. with carbon dioxide created inside the gastrointestinal channel etc., and to avoid direct contact between the pharmaceutically active substance, i.e. the acid-labile compound, and acidic substances of the effervescence, and further to avoid direct contact of the active substance with a solution buffered to unsuitable pH, would be to use the active substance in the form of small enteric coating layered units comprising the pharmaceutically active substance. Such units are coating layered with a polymeric layer not dissolving in the solution formed when the effervescent tablet is dissolved. These small coating layered units are taste-masked as they maintain their coating layer intact during and after intake of the effervescent dispersion and during passage of the stomach. The coating layer starts to dissolve upon arrival at the appropriate place in the gastrointestinal channel, i.e. in the small intestines (duodenum). The present invention now surprisingly provides such enteric coating layered units suitable for an effervescent formulation.
Preparation of a multiple unit tableted dosage form arises specific problems when enteric coating layered pellets containing acid susceptible proton pump inhibitors as active substances are compressed into tablets. If the enteric coating layer does not withstand the compression of the pellets into a tablet the susceptible active substance will be destroyed both by the acidic solution/dispersion formed upon effervescence or by penetrating acidic gastric juice upon administration, i.e. the acid resistance of the enteric coating layer of the pellets will not be sufficient in the tablet after compression.